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288
METALLURGY OF IRON" AND STEEL".
than Ar, but it is also found that in order to induce the change Ar the steel must first be heated past the point Ac. while the change at Ac cannot take place unless the steel has first been cooled to a point below Ar. It is clear therefore that these two .retardations are simply opposite phases of the same phenomena.
The previous discussion has considered only steels containing as
' much as one-half of one per cent, of carbon and mention has been
made of only one critical point, when as a matter of fact it is
quite certain that there are three, although it will be shown later
•that .the three points are practically coincident in steels containing
.50
.60
.70        .80
ABSCISSAS =CARBON CONTENT ,ORDINATES=TEMPERATURE CENT.
FIG. XV-A.—VARIATIONS IN THE CRITICAL POINTS IN DIFFERENT
STEELS.
*
over 0.30 per cent, of carbon. At one of these points, recently proven to be the second, is- the point of magnetic transformation. Below this point carbon steel is attracted by a magnet. Abov,e this point it is attracted only slightly if at all. It has been before explained that the critical points are found at a slightly different temperature according to whether the metal is being heated or being cooled, and it is evident that the point of magnetic transformation, which, coincides with the second critical point, will vary in the same way.
In soft steels these three points are readily distinguished, but as